The present invention relates to a motor-driven brake apparatus suitable for use in a vehicle.
In a conventional hydraulic brake apparatus, the return of a piston of a caliper that presses a brake pad depends on the roll-back action of a piston seal. Accordingly, there may be variations in the clearance between the brake pad and a brake rotor when the brake pad is returned. In such a case, even when the brake pedal is not operated, the brake pad may drag owe to runout or variations in wall thickness of the brake rotor.
The drag of the brake pad causes the brake pad to wear even when the brake pedal is not operated. During running of the vehicle, the drag of the brake pad causes the fuel consumption performance to lower. Thus, the brake pad drag impairs economy and may also cause judder.
To solve the above-described problem, motor-driven brake apparatus has heretofore been proposed as a substitute for the hydraulic brake apparatus. The conventional motor-driven brake apparatus has a motor and a converting mechanism for converting rotational motion of the motor into rectilinear motion of a piston. The motor-driven brake apparatus further has a position detector for detecting a stroke position of the piston, and a controller for controlling the motor on the basis of the result of detection by the position detector. In the motor-driven brake apparatus, a pad is pressed against a disk by the rectilinear motion of the piston to generate braking force. In a vehicle, such a motor-driven brake apparatus is provided for each wheel.
In the conventional motor-driven brake apparatus, the advance and retract positions of a brake pad-pressing member of the caliper, i.e. the advance and retract positions of the brake pad, can be controlled by controlling the drive of an electric actuator. Therefore, the brake pad can be retracted to an initial position where the brake pad has a predetermined clearance with respect to the disk rotor such that the brake pad will not drag when the brake pedal is not operated. Accordingly, the above-described problem associated with the hydraulic brake apparatus can be solved.
However, the advantageous feature of the conventional motor-driven brake apparatus is unfavorable from the viewpoint of brake feeling under the following circumstances.
In the conventional motor-driven brake apparatus, as shown in FIG. 7, when the brake pedal is operated (time A), the electric actuator is driven to move the brake pad from the above-described initial position F to position Z (time B). Therefore, the following problems arise:
(1) The initial response characteristics of the motor-driven brake apparatus degrade by an amount corresponding to a period of time required to reduce the pad clearance. PA1 (2) During the movement of the brake pad for reducing the pad clearance, the brake pad moves from the initial position (where the pad clearance is maintained at the above-described predetermined value) to the zero position (where the brake pad begins to contact the disk rotor) in a state where substantially no resistance is offered to the brake pad, and then rapidly comes into contact with the brake rotor. Therefore, noise is generated when the brake pad contacts the brake rotor. PA1 (3) The presence of the clearance undesirably allows dust or water to enter the space between the brake rotor and the brake pad, resulting in a reduction in the initial braking force, which is applied at the time of initiation of a brake operation. This brings about an adverse effect on the initial response characteristics at the time when the brake pad begins to contact the brake rotor. PA1 (4) When the motor-driven brake apparatus is not used for a long period of time during running of the vehicle, the pad performance of the motor-driven brake apparatus degrades in comparison to a case where the motor-driven brake apparatus is used at moderate frequency.
Furthermore, in the conventional motor-driven brake apparatus, when the brake pedal is not operated, the brake pad is placed at a retract position, that is, the initial position where the brake pad has a predetermined clearance with respect to the disk rotor, as stated above. Therefore, when the motor-driven brake apparatus is not used for a long period of time during running of the vehicle, dust or water collects in the space between the disk and the pad, causing the pad performance of the motor-driven brake apparatus to degrade in comparison to a case where the motor-driven brake apparatus is used at moderate frequency.